1. Technical Field
This disclosure relates to a surface emitting laser element that emits light in a vertical direction with respect to a substrate, a surface emitting laser array including such surface emitting laser elements, and an optical scanning apparatus, an image forming apparatus, and am optical communication system that use light emitted from such surface emitting laser element or surface emitting laser array.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser that emits light in a vertical direction with respect to a substrate. Since the VCSEL does not require a cleavage process, may be integrated into a two-dimensional parallel arrangement, and may be easily inspected, it is suitably used in various consumer appliances such as the light source of an image forming apparatus, the light source of an optical pickup device, the optical communication light source of an optical interconnection, for example.
The VCSEL used in such appliances desirably has an active layer with a large gain, a low threshold value, high optical power, reliability, and adequately controlled polarization. However, since the volume of the active layer of the VCSEL is relatively small, the optical power of the VCSEL may be relatively low compared to an edge emitting semiconductor laser.
Generally, the optical power, optical spectrum, mode, and service life of a semiconductor laser are affected by the degree of thermal diffusion resulting from current injection. It is particularly noted that since the VCSEL has its active layer arranged between semiconductor multilayer reflector (DBR: Distributed Bragg Reflector) mirrors that have high thermal resistance, significant temperature increase may occur in the active layer.
Accordingly, techniques have been proposed for controlling such temperature increase in the active layer of the VCSEL. For example, Japanese Laid-Open Patent Publication No. 2002-164621 discloses a surface emitting semiconductor laser element having a bottom DBR made of AlGaAs material, wherein AlAs, which has the lowest thermal resistance among AlGaAs materials, is used as a lower refractive index layer making up a large portion of the lower side of the bottom DBR.
Japanese Laid-Open Patent Publication No. 2005-354061 discloses a VCSEL that includes a high thermal conductivity layer arranged between an active area and a DBR or within the DBR. The disclosed VCSEL may use AlAs as the high thermal conductivity layer material and further arranges the high thermal conductivity layer to have adequate thickness.